@device-router/middleware-fastify

@device-router/middleware-fastify

Fastify middleware for DeviceRouter. Adds device classification and rendering hints to every request.

Installation

pnpm add @device-router/middleware-fastify @device-router/storage @fastify/cookie

For automatic probe injection:

pnpm add @device-router/probe

Quick start

import Fastify from 'fastify';
import cookie from '@fastify/cookie';
import { createDeviceRouter } from '@device-router/middleware-fastify';
import { MemoryStorageAdapter } from '@device-router/storage';

const app = Fastify();
const { middleware, probeEndpoint } = createDeviceRouter({
  storage: new MemoryStorageAdapter(),
});

await app.register(cookie);

app.post('/device-router/probe', probeEndpoint);
app.addHook('preHandler', middleware);

app.get('/', (req, reply) => {
  const profile = req.deviceProfile;

  if (profile?.hints.preferServerRendering) {
    return reply.send(renderSSR());
  }
  if (profile?.hints.deferHeavyComponents) {
    return reply.send(renderLite());
  }
  reply.send(renderFull());
});

app.listen({ port: 3000 });

How it works

• Probe endpoint receives device signals from the browser and stores a classified profile
• Middleware is a preHandler hook that reads the session cookie, loads the profile from storage, and attaches it to req.deviceProfile
• Your route handlers use req.deviceProfile.hints and req.deviceProfile.tiers to adapt responses

Probe auto-injection

Automatically inject the probe <script> into HTML responses:

const { middleware, probeEndpoint, injectionMiddleware } = createDeviceRouter({
  storage: new MemoryStorageAdapter(),
  injectProbe: true,
  probeNonce: 'my-csp-nonce', // optional
});

When injectProbe is enabled, injectionMiddleware is returned as an onSend hook. Register it to inject the script before </head>:

app.addHook('preHandler', middleware);
if (injectionMiddleware) {
  app.addHook('onSend', injectionMiddleware);
}

Streaming responses: The onSend hook receives the serialized payload as a string. If you stream responses via reply.raw, the hook is bypassed and injection is skipped. Add the probe <script> tag to your HTML shell manually instead.

Custom thresholds

const { middleware, probeEndpoint } = createDeviceRouter({
  storage,
  thresholds: {
    cpu: { lowUpperBound: 4, midUpperBound: 8 },
    memory: { midUpperBound: 8 },
  },
});

Options

Option	Type	Default	Description
storage	StorageAdapter	(required)	Storage backend for profiles
cookieName	string	'device-router-session'	Session cookie name
cookiePath	string	'/'	Cookie path
cookieSecure	boolean	false	Set Secure flag on the session cookie
ttl	number	86400 (24h)	Profile TTL in seconds
rejectBots	boolean	true	Reject bot/crawler probe submissions
thresholds	TierThresholds	Built-in defaults	Custom tier thresholds (validated at startup)
injectProbe	boolean	false	Auto-inject probe into HTML
probePath	string	—	Custom probe endpoint path
probeNonce	string | ((req: FastifyRequest) => string)	—	CSP nonce for injected script
fallbackProfile	FallbackProfile	—	Fallback profile for first requests
classifyFromHeaders	boolean	false	Classify from UA/Client Hints
onEvent	OnEventCallback	—	Observability callback for logging/metrics

Observability

Pass an onEvent callback to receive events for classification, storage, bot rejection, and errors:

const { middleware, probeEndpoint } = createDeviceRouter({
  storage,
  onEvent: (event) => {
    console.log(`[device-router] ${event.type}`, event);
  },
});

See the Observability guide for details.

Standalone usage

Use the individual pieces when you need fine-grained control over each component:

import {
  createMiddleware,
  createProbeEndpoint,
  createInjectionMiddleware,
  loadProbeScript,
} from '@device-router/middleware-fastify';

// Use only what you need
const hook = createMiddleware({ storage, thresholds });
const endpoint = createProbeEndpoint({ storage, ttl: 3600 });
const injection = createInjectionMiddleware({
  probeScript: loadProbeScript(),
});

app.addHook('preHandler', hook);
app.addHook('onSend', injection);
app.post('/device-router/probe', endpoint);

loadProbeScript() reads the @device-router/probe bundle and optionally rewrites the endpoint URL via { probePath }. Thresholds passed to createMiddleware() are validated at creation time.

Exports

• createDeviceRouter(options) — All-in-one setup returning { middleware, probeEndpoint, injectionMiddleware? }
• createMiddleware(options) — Standalone preHandler hook (validates thresholds)
• createProbeEndpoint(options) — Standalone probe endpoint handler
• createInjectionMiddleware(options) — Standalone onSend injection hook
• loadProbeScript(options?) — Load the minified probe script for use with createInjectionMiddleware()

Compatibility

• Fastify 5.x
• @fastify/cookie 11.x
• Node.js >= 20

License

MIT